In certain solid-state imagers, particularly one using a Schottky-barrier diode infrared detector array in combination with a charge-coupled device (CCD) multiplexer to provide for detector scanning, the dynamic range of useful photosensor response is greater than the range of linear response available from any prior-art electrometer of floating-diffusion type. This is also the case with certain visible-light-responsive CCD imagers as well. The prior-art floating-diffusion electrometer is described in United Kingdom Patent No. 1,377,126 published Dec. 11, 1974, issued to RCA Corporation on Apr. 9, 1975, and entitled "CHARGE COUPLED CIRCUITS".
A more recent prior-art floating diffusion electrometer includes a number of insulated gate field effect transistors of metal oxide semiconductor type. The first of these MOSFETs is connected at its gate electrode to the floating diffusion to sense the electrostatic potential induced thereon by a charge packet transferred thereto from the output port of a CCD. This first MOSFET may be operated as a source-follower for providing a electromotive force potential at its source electrode responsive to the electrostatic potential applied to its gate electrode from the floating diffusion, by way of example. Alternatively the first MOSFET may be operated as a common-source amplifier for supplying a drain current responsive to the electrostatic potential applied to its gate electrode. The floating diffusion is the virtual source electrode of a second, plural-gate electrode MOSFET having a drain electrode connected to a reference potential. The gate electrode of this second MOSFET closest to the floating diffusion is connected to a fixed gate potential. A gate electrode of this second MOSFET more remote from the floating diffusion is pulsed after each charge packet in the floating diffusion has had its amplitude sensed. This is done to render the channel of the second MOSFET conductive between the floating diffusion and its drain diffusion contacted by its drain electrode for connection to the reference potential. The conduction of the second MOSFET channel clamps the floating diffusion to reference potential and drains away the charge packet in the floating diffusion. This prepares the floating diffusion to accept the next charge packet transferred thereto by the CCD.
Experience with a Schottky-barrier diode detector array with CCD multiplexer and floating-diffusion electrometer has been as follows. The source-follower MOSFET in the floating diffusion electrometer has an acceptably linear dynamic range of 2 volts at its source electrode. The charge-to-voltage conversion factor of the source-follower is approximately one microvolt per electron. Thus, acceptably linear electrometer response is limited to 2.multidot.10.sup.6 electrons. The Schottky-barrier diode detectors and CCD multiplexer can be designed with charge capacities in the range of 5.multidot.10.sup.6 to 10.sup.7 electrons for practical pixel sizes and CCD multiplexer dimensions, however. It is desirable then to be able to increase the linear dynamic range of the floating diffusion electrometer by a factor of 2.5 to 5.
This is difficult to do. However, the wide dynamic ranges are seldom fully utilized in any imaging task. Rather, linear dynamic range is usually desired both for intense images and for weak images. It is possible then, particularly when an iris is used in the image optics, to achieve the benefits of very wide dynamic range by providing two less-wide linear dynamic ranges which together provide a piecewise-linear wide dynamic range. One may, of course, provide a larger number (e.g. three) of such less-wide linear dynamic ranges, to more closely piecewise approximate the results achievable with a very wide dynamic range.
To provide a plurality of dynamic ranges, one arranges to alter the electrometer sensitivity--i.e., the conversion ratio from floating diffusion charge to source-follower MOSFET source voltage. To understand the way this is done, consider the following equation descriptive of the conversion ratio. EQU V.sub.OUT /Q.sub.SIG =A.sub.V /C.sub.FD
where:
V.sub.OUT =the source-follower source voltage swing responsive to the signal charge packet on the floating diffusion, PA1 Q.sub.SIG =the quantity of charge (in coulombs) in the signal charge packet, PA1 A.sub.V =the source-follower voltage gain, and PA1 C.sub.FD =the floating diffusion capacitance (in farads) including source-follower gate capacitance.
One can attempt to alter the conversion ratio by altering either A.sub.V or C.sub.FD.
In an article entitled "A Controllable Piecewise Linear-Output Circuit for CCD Multiplexers" appearing in the August 1985 "IEEE TRANSACTIONS ON ELECTRON DEVICES" Vol. ED-32, No. 8, Pages 1538-1540, S. B. Stetson describes altering the conversion ration by selectively connecting additional capacitance to the floating diffusion. This connecting is done through the channel of a further MOSFET responsive to control signal applied to its gate electrode.
A substantially simpler structure than that described by Stetson will provide a piece-wise linear, very wide dynamic range, the present inventor finds.